This invention relates generally to ultrafiltration technology, and more particularly to a method for sealing components of spiral wound filtration modules for use in cross-flow filtration.
The term "ultrafiltration" as used in the present application is intended to encompass microfiltration, nanofiltration, ultrafiltration, reverse osmosis and gas separation. A typical ultrafiltration device comprises a plurality of spiral wound filtration modules through which a fluid to be filtered passes.
Such a spiral wound filtration module consists of leaves in which a layer of a permeate carrier material, usually a porous felt or fabric material sold under the tradename TRICOT, is sandwiched between two membrane sheets. The membrane sheets comprise a membrane material integrally joined to a backing material. The membrane sheets are oriented relative to the permeate carrier material so that the membrane material is facing outwardly. The membrane sheets are typically folded once along their length to present a leaf with two halves integrally joined. The outside edges of the leaves are then sealed on all but one side, allowing access to the permeate carrier from a radial direction through the membrane. The membrane module is made by winding one or more membrane leaves around a permeate tube which has holes therein to collect the filtered product, or permeate. The membrane leaves are placed with the unsealed edge of each leaf adjacent to the permeate tube and oriented along the length of the permeate tube, allowing the permeate to flow into the permeate tube.
Each membrane leaf is separated from adjacent leaves by feed spacer screens, which are of a relatively large mesh size to accommodate feed fluid flow. The membrane leaves and feed spacer screens are spirally wound around the permeate tube. After the leaves are wound, some type of external restraining means such as a hard shell, straps or a bypass screen, or a combination thereof may be used to hold the spirally wound leaves in tight formation around the tube. The spiral module is then loaded into a pipe-like housing or pressure vessel which is operated at a slight pressure drop across the module as the fluid being filtered flows through.
In use, the product to be filtered is introduced under pressure at one end face of the module and is allowed to travel axially along the module through the feed spacer screens. Because the outside edges of the membrane leaves are sealed, the feed fluid is prevented from entering into the permeate carrier material without first passing through a membrane sheet. As the feed fluid flows axially through the module along the feed spacer screen, the permeate is allowed to pass through the membrane sheets and will be directed to the permeate carrier tube by the permeate carrier sheet. Concentrate is removed from one end of the module and permeate is removed through the permeate tube. This type of filtering, with a spiral wound module, is advantageous for a number of different applications. However, the manufacturing of these modules presents certain difficulties.
When winding a spiral wound filtration module, the layers of the leaves and feed spacer screens must be able to slide relative to one another, because the outside layers will be required to travel a greater distance than the inside layers due to the increasing circumferential distance. Therefore, it is now the practice of most manufacturers to use a wet adhesive, usually a two part epoxy or urethane, to seal the outer edges of the membrane leaves against one another, with the permeate carrier material sandwiched therebetween. This adhesive is either applied to the permeate carrier material or to the back sides of the membrane sheets, or both, as they are positioned one on top of the other. The spiral is then wound while the adhesive is wet, and the adhesive is allowed to cure after the spiral is wound. An additional bead of adhesive is applied across the bottom of each leaf axially along the module, from one side seam to the other. This axial bead can be applied either before or after winding the module.
Sealing the outer edges of a spiral wound module by application of a wet adhesive prior to winding presents many disadvantages. For example, the adhesive application process is now performed manually. As such, the process is labor intensive. This labor intensive process is compounded when a module having a large number of leaves is manufactured. Typically, a minimum of three and up to as many as twenty leaves are present in a spiral wound module.
Another disadvantage of the application of wet adhesive prior to winding is that the resulting seams between the membrane sheets and the permeate carrier material tend to be uneven due to a lack of automated adhesive application control. Uneven spreading of the glue, which takes place as the module is wound and tightened, further contributes to uneven seams. Uneven seams means that the seams are wider than necessary in some areas, which wastes potential active membrane area. In other words, any membrane area that is sealed beyond that necessary eliminates usable membrane surface area. Still further, the uneven seams make it difficult to clean the module to the level desired in sanitary applications. It is more difficult to properly clean a module having uneven and irregular seams than a module having consistent and straight seams.
Further, using the current wet adhesive method, care must be taken in winding to ensure that the adhesive is sheared so that the layers slide relative to one another. If such care is not taken, wrinkles may result in one or more of the materials, leading to a defective membrane module. It is difficult, however, to avoid wrinkling during wind up when using a wet adhesive due to the friction created by the uncured adhesive on the materials. Further, when winding a membrane module using a wet adhesive, the two outside edges of the layers must be tensioned uniformly to ensure that the same diameter is formed across the module, and to attempt to get the layers to lie flat so as to achieve a tight channel during winding. With the adhesive in place, using the current method, it is difficult to properly tension the materials and to get the many layers to lie flat so that a uniform, tight channel is produced during winding. This difficulty in applying uniform tension to the layers may result in modules with seams that have air voids or leaks, which require repair.
A further drawback in the use of the current wet adhesive method is that the adhesives used must have a viscosity high enough to resist running out of the relatively open crevices which are present near the edges of the membrane leaves. This high viscosity, however, makes it more difficult to achieve sufficient adhesive penetration into the fine openings in the backing of the membrane sheets. Without proper adhesive penetration, the seal between the membrane sheets and the permeate carrier can be defective. It is therefore difficult to balance the need for a high viscosity adhesive against the need for an adhesive which adequately penetrates into the backing of the membrane sheets to sufficiently hold the layers together.
Therefore, a device is needed which overcomes the drawbacks and deficiencies of the existing constructions and methods discussed above.